Field of the Invention
The invention concerns a method for producing a cold-rolled steel strip with an optimised yield ratio Re/Fm and a correspondingly obtained cold-rolled flat steel product.
Description of Related Art
Flat steel products of the kind involved here are rolled products obtained by cold rolling such as steel strips or sheets, and blanks and plates made from these.
Unless expressly indicated to the contrary all particulars of content of the steel compositions indicated in this application refer to weight. All indications in “%” not specified in more detail in relation to a steel alloy must therefore be understood to be in “wt. %”.
Details of structural constituents in each case refer to percent by volume (“vol.-%”), unless expressly indicated to the contrary.
High-strength flat steel products are becoming increasingly important, in particular in the area of commercial vehicle construction, because they allow a reduction in the vehicle's own weight and an increase in the useful load.
A lower weight contributes not only to optimal utilisation of the technical performance of the respective drive unit, but aids resource efficiency, cost optimisation and climate protection.
A significant reduction in the unladen weight of sheet steel constructions can be achieved by an increase in the mechanical properties, in particular the strength of the flat steel product used in each case.
Apart from high strength, however, modern flat steel products intended for commercial vehicle construction are also expected to be tough and have good brittle facture resistance behaviour and optimum suitability for cold working and welding.
There have been a large number of trials aimed at meeting these requirements through alloying or process engineering. A common feature of these trials is that they were based on a so-called two or multiphase steel, the structure of which in each case comprised at least two dominant phases, wherein for multi-phase steels lower contents of other phases may be present.
Thus for example from WO 2013/082171 A1 a flat steel product comprising a two-phase steel, with a ferritic-martensitic structure is known. The flat steel product comprises a steel containing 0.5-3.5 wt. % Si, 0.1-0.3 wt. % C, 1-3 wt. % Mn, and in each case optionally 0.05-1 wt. % Al, a total of 0.005-0.1 of one or more of the elements Nb, Ti und V and up to 0.3 wt. % Mo, and as a result has a tensile strength of at least 980 MPa and an elongation of at least 15%. The flat steel product is produced in an in itself known manner by hot and cold-rolling. After cold-rolling it undergoes heat treatment, in which it is initially annealed at a temperature of 775-825° C., then it is quenched before undergoing ageing treatment at 200-420° C. for a period of 150 seconds.
From DE 10 2012 013 113 A1 a cold-rolled flat steel product is also known, comprising a high-strength, multi-phase steel with minimum tensile strengths of 580 MPa, containing 0.075-0.105 wt. % C, 0.6-0.8 wt. % Si, 1.0-2.25 wt. % Mn, 0.28-0.48 wt. % Cr, 0.01-0.6 wt. % Al, up to 0.02 wt. % P, up to 0.01 wt. % N, up to 0.015 wt. % S and as the remainder iron and unavoidable impurities. During the process, following cold-rolling, the flat steel product undergoes heat treatment comprising a complete temperature cycle. Thus the strip is initially heated to a target temperature of 700-950° C., and then cooled at a cooling rate of 10-100° C./s to an intermediate temperature of 300-500° C., then again at a cooling rate of 15-100° C./s to a second intermediate temperature of 200-250° C. and finally at a cooling rate of 2-30° C./s to ambient temperature. Through this measure an optimised structure and associated optimised mechanical properties are to be arrived at.
EP 2 551 359 A1 discloses a method for manufacturing an ultrahigh strength member having a tensile strength of 1180 MPa or more and showing an excellent delayed fracture resistance. To achieve this, a steel sheet is heated at a first heating temperature of 700-1000° C., formed into a shape of a member at the first heating temperature and simultaneously cooled. After completion of the cooling, the obtained member is shear punched into a desired shape to obtain an ultrahigh strength member. Then the ultrahigh strength member is subjected to first heat treatment including heating and retaining the member held at a second heating temperature of 100 to less than 300° C. for 1 second to 60 minutes.
According to US 2011/0048589 A1 an ultra-high strength steel sheet can be obtained on the basis of a steel which comprises 0.12-0.50 wt.-% C, up to 2.0 wt.-% Si, 1.0-5.0 wt.-% Mn, up to 0,1 wt.-% P up to 0.07 wt.-% S, up to 1.0 wt.-% Al, up to 0.008 wt.-% N, balance Fe and incidental impurities. The steel's micro-structure includes, on an area ratio basis, 80% or more of autotempered martensite, less than 5% of ferrite, 10% or less of bainite, and 5% or less of retained austenite. To adjust the autotempered martensite fraction in the microstructure the steel sheet passes after cold rolling an annealing treatment in the course of which it is annealed for 15-600 seconds in a first temperature range which lower limit is defined by the Ac3 transformation temperature of the respective steel and which upper limit is set to 1000° C. Specifically, the temperature of the annealing is adjusted such that the micro structure of the steel exclusively consists of austenite. The annealed cold rolled steel sheet is then cooled with a cooling rate of 3° C./s or higher to a temperature range ranging from 780° C. to the Ac3-temperature. After that temperature range is reached the steel sheet is cooled to 550° C. When this temperature is passed the cold-rolled steel sheet is subjected to an autotempering treatment in the course of which the steel sheet is cooled down to a temperature range of 150-300° C. with a cooling rate of 0.01-10° C./s.